The invention relates to a gas and steam turbine plant with a waste-heat steam generator which is located downstream of a gas turbine on the flue-gas side and which has heating surfaces that are connected into a water/steam circuit of a steam turbine. A fuel gasification device is located upstream of a combustion chamber of the gas turbine through a fuel line.
A gas and steam turbine plant with integrated gasification of fossil fuel conventionally includes a fuel gasification device which is connected on the outlet side to the combustion chamber of the gas turbine through a number of components provided for gas purification. In that case, a waste-heat steam generator is connected downstream of the gas turbine, on the flue-gas side. Heating surfaces of the waste-heat steam generator are connected into the water/steam circuit of the steam turbine. A plant of that type is known, for example, from UK Patent Application GB 2 234 984 A or from U.S. Pat. No. 4,697,415.
An apparatus for the removal of sulfur-containing constituents is provided, in both plants, for the reliable purification of the gasified fossil fuel. In the plant known from UK Patent Application GB 2 234 984 A, a saturator for inerting the fuel gas is located downstream of that apparatus in a supply line for the gasified fuel which opens into the combustion chamber. The gasified fuel is laden with steam in that saturator in order to reduce pollutant emissions. For that purpose, the gasified fuel flows through the saturator in countercurrent to a water stream which is carried in a water circuit referred to as a saturator circuit. A provision is made for feeding heat from the water/steam circuit into the saturator circuit in that case, in order to operate the saturator independently of the gas generation or gas purification plant.
That plant is intended to operate with gasified coal or gasified refinery residues, for example residual oil, as fossil fuel and is therefore also adapted to process properties for the gasification of coal or of residual oil with a view toward achieving particularly high efficiency. In particular, the plant is constructed in terms of the water/steam circuit of the steam turbine, with a view toward cost-effective and operationally reliable utilization of the heat occurring during gasification.
It is accordingly an object of the invention to provide a gas and steam turbine plant, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type, which has a particularly simple structure and in which both high plant efficiency and independent and simple-to-regulate operation for inerting fuel gas are ensured, even when oil is used as a fossil fuel.
With the foregoing and other objects in view there is provided, in accordance with the invention, a gas and steam turbine plant, comprising a gas turbine having a flue-gas side. A combustion chamber which is connected to the gas turbine has a water/steam circuit. A waste-heat steam generator is disposed downstream of the gas turbine on the flue-gas side. The steam generator has heating surfaces connected into the water/steam circuit. A fuel line is connected to the combustion chamber. A gasification device for producing gasified fuel is connected into the fuel line, upstream of the combustion chamber. A mixing apparatus is connected into the fuel line for admixing nitrogen to the gasified fuel. A heat exchanger has a primary side connected into the fuel line, upstream of the mixing apparatus, as seen in a flow direction of the gasified fuel. The heat exchanger has a secondary side constructed as an evaporator for a flow medium, as well as a steam side connected to the combustion chamber.
The invention disclosed herein is based on the concept that for high plant efficiency, even when oil is used as a fossil fuel, particularly effective utilization of the heat carried in the fuel stream flowing off from the gasification device, which is also referred to as crude gas, should be provided. At the same time, precisely when oil is used as a fossil fuel, it should be kept in mind that a large part of the crude-gas heat may occur in the form of latent heat as a result of partial water condensation at comparatively low temperature. It is precisely this heat which can be extracted from the crude-gas stream in a particularly advantageous way by the evaporation of a flow medium. The flow medium is capable of being fed into the plant process at a suitable point in a particularly simple and flexible way. In addition, and for permitting the inerting system for the fuel gas to operate independently of the water/steam circuit of the steam turbine located downstream of the gas turbine, with a suitable choice of the pressure level, the steam that is generated can be fed directly as an inerting medium to the fuel gas or to the GT burner. In this case, through the use of the heat exchanger, particularly favorable operating parameters, in particular a particularly favorable temperature level, of the crude gas can be established for the subsequent mixing of the crude gas with nitrogen. This mixing is intended for the purpose of adhering to particularly low NOx limit values.
Supplying the steam generated in the heat exchanger into the fuel stream makes it possible to fully ensure that the gasified fuel is laden with steam sufficiently to adhere to even low pollutant emission limit values. Therefore, complicated devices normally provided for loading the gasified fuel with steam may be dispensed with completely. In particular, a gas and steam turbine plant of this type can be constructed so as to dispense with the saturator which is normally provided, together with the further components associated therewith, so that a particularly simple structure is obtained. Moreover, feeding the evaporated flow medium into the combustion chamber of the gas turbine ensures that the heat extracted from the crude gas during the evaporation of the flow medium is utilized particularly effectively for the plant process. The apparatus also allows simple and operationally reliable regulation of the steam content of the fuel gas in order to adhere to the predetermined limit values for NOx emission.
In accordance with another feature of the invention, the heat exchanger is constructed as a medium-pressure evaporator for water as the flow medium. In this case, the heat exchanger is constructed preferably for evaporating the water at a pressure stage of about 20 to 25 bar. Thus, medium-pressure steam generated in this way and not required to be fed into the combustion chamber can also be utilized in a particularly advantageous way for the plant process and may, for example, be fed into the water/steam circuit of the steam turbine.
In accordance with a further feature of the invention, the heat exchanger is connected to a low-pressure stage of the water/steam circuit of the steam turbine on the steam side through a branch line, into which a shut-off member and a throttle apparatus are connected. In this case, the gas and steam turbine plant may be constructed in such a way as to ensure that a steam quantity which is sufficient for adhering to predetermined pollutant emission limit values and which is to be supplied to the fuel is produced in every operating state. Thus, after throttling, possibly excess steam generated in the heat exchanger can be utilized directly for energy generation in order to achieve particularly high efficiency in the low-pressure stage of the water/steam circuit. Conversely, if the NOx emission requirements are particularly stringent, additional medium-pressure steam from the water/steam circuit may also be admixed, preferably upstream of the intermediate superheater of the waste-heat boiler.
In a further advantageous refinement, the heat exchanger for medium-pressure steam generation has a further heat exchanger for low-pressure steam generation disposed downstream thereof, so that the maximum fraction of crude-gas heat at low temperature can be utilized with high efficiency. The generated steam, together with the throttled medium-pressure steam, may be delivered to the low-pressure part of the water/steam circuit. A further heat exchanger for cooling the crude gas may be provided, depending on the gas purification requirements, in particular the temperature level of possibly downstream COS hydrolysis.
In accordance with a concomitant feature of the invention, for particularly high plant efficiency, there is provided a crude-gas waste-heat steam generator preceding the medium-pressure evaporator in the fuel line upstream of the heat exchanger. Through the use of the crude-gas waste-heat steam generator, it is possible for the crude gas or synthesis gas generated in the gasification device to be precooled as required and in a manner which is advantageous in material terms.
The advantages achieved through the use of the invention are, on one hand, in particular, that even when oil is used as a fossil fuel, particularly high overall efficiency of the plant can be achieved. Utilizing the heat which is carried in the crude gas and which may, in particular, take the form of latent heat at a comparatively low temperature level in order to evaporate the flow medium, makes it possible to supply this heat into the plant process in a particularly effective and flexible way. Particularly when water is evaporated as the flow medium and this steam is subsequently fed into the mixed gas, it becomes possible for the mixed gas to be sufficiently laden with steam, even without connecting a saturator which per se, together with the further components associated therewith, would entail a significant outlay in terms of manufacture and assembly. On the other hand, admixing the steam makes it possible to set the degree of saturation of the fuel gas over a wide parameter range and to provide a simple and quick-reacting concept for regulating the steam content. This ensures that even low limit values for pollutant emission are adhered to at a particularly low outlay.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a gas and steam turbine plant, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.